1. Field of the Invention
The present invention relates to an optical fiber chromatic dispersion distribution measuring apparatus for measuring the chromatic dispersion distribution of an optical fiber and a measuring method.
2. Description of the Related Art
It is known that when two pulse light beams having different wavelengths xcex1, xcex2 from each other are simultaneously inputted to an optical fiber under test, four-wave mixing light beams are generated due to interaction between the two inputted pulse light beams.
A relation among the wavelengths xcex1, xcex2 of the pulse light beams and wavelengths xcex3, xcex4 of the four-wave mixing light beams is shown in FIG. 2.
In FIG. 2, the longitudinal axis indicates the wavelength of each of light beams and the transverse axis indicates the intensity of each of light beams. Symbols a and b indicate the pulse light beams having the wavelength xcex1 and xcex2, respectively. Symbols c and d indicate the four-wave mixing light beams having wavelengths xcex3 and xcex4, respectively. The wavelengths xcex1, xcex2, xcex3, and xcex4 satisfy the following relation:
xcex1xe2x88x92xcex3=xcex4xe2x88x92xcex2=xcex2xe2x88x92xcex1=xcex0 (xcex0 is about 5 to 10 nm)
An interval between the wavelengths of the pulse light beams (that is, xcex2xe2x88x92xcex1=xcex0) is the smaller, the intensity of the four-wave mixing light beams are the larger.
An optical fiber chromatic dispersion distribution measuring apparatus according to a related art extracts either one of the four-wave mixing light beams having the wavelengths xcex3 and xcex4, that are reflected from the optical fiber under test, by an optical bandpass filter having a variable center wavelength to execute measurement of the chromatic dispersion distribution of the optical fiber under test.
However, due to a mechanical structure of the optical bandpass filter having the variable center wavelength, a loss caused by inserting the optical bandpass filter having the variable center wavelength is more than 10 dB to decrease the measurement sensitivity.
In case of compensating the loss, which is caused by inserting the optical bandpass filter having the variable center wavelength, by using an optical amplifier, the configuration of the apparatus becomes complicate.
An object of the invention is to provide an optical chromatic dispersion distribution measuring apparatus having a simple structure, enabling to change an interval of wavelengths of two inputted pulse light beams, and having high measurement sensitivity.
According to a first aspect of the invention, there is provided an optical fiber chromatic dispersion distribution measuring apparatus comprising:
two light sources for outputting light beams having different wavelengths from each other, respectively, to an optical fiber under test;
an optical time domain reflectometer for measuring four-wave mixing light beams generated by an interaction of the light beams inputted to the optical fiber under test; and
an optical bandpass filter having a fixed center wavelength,
wherein at least one of the two light source is a tunable light source;
the optical bandpass filter is disposed at a previous stage of the optical time domain reflectometer.
According to a second aspect of the invention, there is provided an optical fiber chromatic dispersion distribution measuring apparatus comprising:
two light sources for outputting CW light beams having different wavelengths from each other, respectively;
an optical coupler for combining a plurality of light beams;
a modulator;
an optical fiber amplifier;
a directional coupler;
an optical fiber under test;
an optical bandpass filter having a fixed center wavelength; and
an optical time domain reflectometer;
wherein at least one of the two light source is a tunable light source;
the two light sources output the CW light beams to the optical coupler;
the optical coupler combines the CW light beams and outputs the combined CW light beams to the modulator;
the modulator modulates the CW light beams inputted from the optical coupler to generate pulse light beams having different wavelengths from each other and outputs the pulse light beams to the optical fiber amplifier;
the optical fiber amplifier amplifies the pulse light beams and outputs the amplified pulse light beams to the directional coupler;
the directional coupler outputs the pulse light beams inputted from the optical fiber amplifier to the optical fiber under test and outputs a light beam inputted from the optical fiber under test to the optical bandpass filter;
four-wave mixing light beams are generated in the optical fiber under test due to an interaction of the light beams inputted from the directional coupler and is outputted to the directional coupler;
the optical bandpass filter extracts a light beam within a specific band from the light beam inputted from the directional coupler and outputs the extracted light beam to the optical time domain refelectometer; and
the optical time domain reflectometer measures the chromatic dispersion distribution of the extracted light beam.
According to a third aspect of the invention, there is provided the optical fiber chromatic dispersion distribution measuring apparatus according to the second aspect of the invention wherein the four-wave mixing light beams are a light beam generated in lower frequency side than the pulse light beams and a light beam generated in higher frequency side than the pulse light beams; and
only one of the four-wave mixing light beams is within the specific band of the optical bandpass filter.
According to a fourth aspect of the invention, there is provided an optical fiber chromatic dispersion distribution measuring method comprising the steps of:
outputting two light beams having different wavelengths from each other, respectively, to an optical fiber under test;
generating two four-wave mixing light beams in the optical fiber under test;
measuring one of the two four-wave mixing light beams to obtain the chromatic dispersion distribution of the optical fiber under test. Thus, even if the wavelengths of the light beams from the two light sources are changed, the optical fiber dispersion can be measured without lowering the measurement sensitivity.
According to a fifth aspect of the invention, there is provided an optical fiber chromatic dispersion distribution measurement method comprising the steps of:
outputting two CW light beams having different wavelengths from each other;
combining the CW light beams;
modulating the CW light beams to generate two pulse light beams having the different wavelengths from each other;
amplifying the pulse light beams;
inputting the pulse light beams to an optical fiber under test to generate two four-wave mixing light beams;
extracting one of the four-wave mixing light beams; and measuring the one of the four-wave mixing light beams to obtain the chromatic dispersion distribution of the optical fiber under test.
According to a sixth aspect of the invention, there is provided the method according to the fifth aspect of the invention, further comprising the steps of adjusting both wavelengths of the two light beams so that wavelength of the one of the four-wave mixing light beams coincides with a center wavelength of an optical bandpass filter having a fixed center wavelength for executing the extracting step. Thus, the wavelength of the four-wave mixing light beam is matched to the center wavelength of the optical bandpass filter more freely.
According to a second aspect of the invention, there is provided the method according to the sixth aspect of the invention, wherein interval between the wavelengths of the two CW light beams is held in the adjusting step. Thus, adjustment can be done while holding the intensity of the four-wave mixing light beam constant.
According to an eighth aspect of the invention, there is provided the method according to the fifth aspect of the invention, wherein ratio of the intensity of the two CW light beams is approximately 2:1. Whereby the optical fiber wavelength dispersion can be measured without any measurable variations in frequency under observation.